Experimental Procedure

Estimated time to complete the experiment: 4 h.

Materials

Reagents

1 pH meter

AI2O3

1 1-mL graduated pipet

Ti02

1 2-mL graduated pipet

MgO

1 5-mL graduated pipet

ZnO

5 20-mL beakers

Si02

1 magnetic stirrer

0.1 MKNO3

1 microburet

0.01 M KOH

1 universal stand

D. I. water

2 10-mL syringes

0.01 M HNO3

1 propipet bulb

Use the following procedure for each oxide. Commercial oxide reagents can be used. Aluminum oxide for column chromatography can also be used. Make sure to stir for at least 1 minute after every experimental step before taking and recording the pH measurements that involve such oxides. The longer the time allowed before a measurement, the closer the system will be to the true equilibrium. Depending on the accuracy desired, the experiments may be replicated as necessary.

(a) pzc determination of Ti02, MgO, ZnO and AI2O3

Prepare two solutions by mixing 3.0 mL of 0.1 M KNO3 and 6.0 mL of deionized (D.I.) H20 in each of two 20-mL beakers. Measure the pH of either one. The first one will be called the blank and the other, the sample. Add to the blank 1.0 mL of 0.01 M KOH and measure the pH. Add to the sample 50 mg of the solid oxide and measure the resulting pH. Add 1.0 mL of 0.01 M KOH to the sample and measure the pH. Titrate the blank with HNO3 0.01 M, and record the resulting curve. Do the same for the sample, and plot both curves in the same graph.

(b) pzc determination of Si02

The case of Si02 is different to those analyzed above due to its acidic nature, which increases the amount of OH- required to deprotonate the initial sites. In addition, the ratio of the surface acidity of Si02 to its acidity in aqueous solution is much lower than that for many other solid oxides. This means that more OH- are required than normal to neutralize its aqueous suspensions. To determine the pzc of Si02, prepare two solutions by mixing 3.0 mL of 0.1 M KNO3 and 5.0 mL of D.I. H20 in each of two 20-mL beakers. Measure the pH of either one. Add to the blank 2.0 mL of 0.01 M KOH and measure the pH. Add to the sample 50 mg of the solid oxide and measure the pH. Then, add 2.0 mL of 0.01 M KOH to the sample and measure the pH. Titrate the blank with HNO3 0.01 M, and record the resulting curve. Do the same for the sample, and plot both curves in the same graph.

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